A mortal sin has three requirements, all three of which must be met for it to be considered mortal:
- The sin must be grave matter
- You must know the act is sinful
- You must voluntarily choose to do the sinful act anyway despite knowing that it is sinful
As you say that you didn’t know about or remember the rule of the Communion fast, that means you most likely did not commit a mortal sin but at worst committed a venial sin. If your ignorance was through no fault of your own, ie the fault of another rather than negligence on your part, then the guilt for that sin may not even have been imputable to you at all.
But consider what it means to fulfill all three requirements for a mortal sin. First it has to be a sin of grave matter. Having a smoothie isn’t a sin of grave matter; absolutely speaking it isn’t sinful at all. Having a smoothie at a time the Church has instructed the faithful to not eat/drink anything but water and medication because our minds should be directed to the heavenly rather than the worldly is a different thing entirely. Such an act is disobedience of the Church’s moral authority granted to her by Jesus Christ. That disobedience is, objectively speaking, grave matter.
But of course grave matter alone isn’t sufficient. You must know that what you’re doing is wrong. God doesn’t hold us accountable for what we cannot know. If we don’t know something is wrong because those charged with instructing us in such matters have failed in their duty, then no guilt attaches to us. If we don’t know that something is wrong because we were told about it years ago but have honestly forgotten due to having imperfect human memories and never being reminded since then, no guilt attaches to us. God is perfectly merciful. But God is also perfectly just. He will hold us accountable for what we do not know if we SHOULD know it. If we don’t know because we willfully avoid opportunities to learn about or be reminded of our obligations as Catholics, then our ignorance is our fault and comes from an attitude of defiance rather than imperfect memory. My priest mentions the Communion fast every 2-3 months, either as a part of his homily or during the announcements at the very end of Mass. The only way to miss out on the reminder is to not attend Mass regularly, which is itself a sin of grave matter. I can’t speak for other parishes, but no one at my parish can realistically be considered ignorant of that particular rule if they’ve been a member for at least 6-9 months.
Then finally you must choose to do it while knowing it is sinful. Simply choosing to do it isn’t enough. You must say, to at least some degree, “I know the Church teaches that X is wrong, but I’m going to do it anyway”.
Note that the common thread in all aspects of a mortal sin is pride. I know better. I am above the law. The rules don’t apply to me. I will do whatever I want simply because I want to do it. That attitude is why a mortal sin is so bad. It’s not simply “Oops” or “I guess I wasn’t really thinking” or “Sorry, I forgot”. Those are venial sins at most. Mortal sins are a completely different degree of sin. A mortal sin is a voluntary twisting of our internal disposition away from others, especially God, and inward on the self.
What’s sacrilegious isn’t any purely physical action the world may see, such at eating less than an hour before receiving Communion. It’s an internal disposition of pride and arrogance. As you say, God sees our hearts. He can clearly see the pride and arrogance within us whenever we have committed a mortal sin, and that pride and arrogance are what offends God. When we turn away from pride and toward humility, bowing to God’s will and humbly accepting sacramental confession, repenting for the sins we have committed and acknowledging the need for forgiveness, we open our heart once more to God’s grace. Our disposition changes from being twisted inward on the self back to being aimed outward toward others, especially God. Then and only then are we properly disposed to receive our Lord in Communion.
On rare occasions I cry after receiving Communion because the joy I feel at receiving our Lord is sometimes more than I can contain. That certainly never happened when I was living with mortal sin. When the burden of moral sin was on my soul, reception of Communion was nothing more to me than a rote, mechanical process.